The present invention relates to a semiconductor switching circuit using a P-channel MOSFET (metal-oxide-semiconductor field effect transistor), and more particularly to an output circuit which includes a P-channel MOSFET having a high withstand voltage and capable of delivering a large current, and which can be readily formed by semiconductor integrated circuit technology.
In recent years, the automobile has been provided with a semiconductor relay to reduce the weight of wiring harness thereof for use as an electronic switch. The combination of a switch and a load can be classified into two cases, that is classification depends on the basis of the positional relation between the switch and the load. According to a first case, the switch acts as a current source as shown in FIG. 7A. According to the second case, the switch acts as a current sink as shown in FIG. 7B. When a positive power supply terminal is placed on the upper side and a ground terminal is placed on the low side as shown in FIGS. 7A and 7B, the switch of FIG. 7A is called "high-side switch", since a switch 1 is placed on the upper side or power supply terminal side of a load 3. On the other hand, the switch in the configuration of FIG. 7B is called a "low-side switch" since the switch 1 is placed on the under side (or ground side) of the load 3. In the high-side switch of FIG. 7A, one end of the load 3 is grounded and in the low-side switch of FIG. 7B, one end of the load is connected to the high potential terminal of a D.C. power source 2. Hence, the high-side switch of FIG. 7A is generally used.
An electronic switch is usually provided with an excess-current preventing circuit for turning off the electronic switch automatically when a current flowing through the electronic switch exceeds a predetermined value. One of objects for replacing a mechanical relay by an electronic switch is to have such a protecting function. In order to perform the protecting function, it is necessary to compare the current flowing through the electronic switch with a reference value. A conventional comparator circuit is operated on a supply voltage measured from a ground potential, to produce a logic output. An output voltage corresponding to an output current which flows through the high-side switch is a floating voltage dependent on a power supply potential V.sub.DD since one end of the high-side switch is connected to the power supply terminal kept at the power supply potential V.sub.DD. Accordingly, it is necessary to convert the output voltage from the high-side switch into a voltage output measured from the ground potential. This conversion is not easy. For this reason, a detected current has hitherto been compared with a reference current independently of an electric potential. That is, an output current flowing through the high-side switch (that is, output MOSFET) is converted into a small, detected output current with the aid of a current mirror current, and the small, detected output current thus obtained is compared with a reference current. A ratio of the small, detected output current to the output current flowing through the output MOSFET (namely, high-side switch) is given by the following equation: ##EQU1## where S.sub.O indicates the gate area of an output MOSFET, and S.sub.D the gate area of a small-sized MOSFET for obtaining the detected output current.
A typical example of the current mirror circuit is shown in FIG. 6 on page 35 of the intelligent motion section of the abstracts of the 88-International Conference of PCIM (held in Tokyo in December, 1988). This current mirror circuit is made up of an output MOSFET and a small-sized MOSFET in such a manner that one of the source and drain of the output MOSFET is directly connected to a corresponding one of the source and drain of the small-sized MOSFET, and the gate of the output MOSFET is connected directly to the gate of the small-sized MOSFET.
In a case where the low-side switch of FIG. 7B is used, a resistor is connected to the source terminal of the small-sized MOSFET as shown in FIG. 1 on page 31 of a Motorola technical report entitled "Lossless Current Sensing with SENSEFETs Enhances Motor Drive" Design published in 1986 (reprinted with permission of POWERCONVERSION & INTELLIGENT MOTION, April 1986 issue), to convert the detected output current into a voltage measured from a ground potential.
Now, let us consider a case where a current flowing through the high-side switch of FIG. 7A is detected with the aid of a current mirror circuit. The high-side switch is mainly applied to an automobile, and hence a lead storage battery is used as a D.C. power source. In an ordinary passenger car, the output voltage of the lead storage battery varies in a range from 12+4 V to 12-4 V. It is very difficult to keep the current mirror ratio of the equation (1) and a reference current constant independently of such variations in output voltage of the D.C. power source.
Further, when the current detection method used in the low-side switch of FIG. 7B is applied to the high-side switch, that is, a resistor is connected to the source terminal of a small-sized MOSFET, the detected voltage becomes a floating voltage dependent on a supply potential V.sub.DD, and thus it becomes difficult to adequately protect the high-side switch from an excess current.